1. Field of the Invention
The present invention relates to an engine start type VSCF generating system suitable for use as an electrical power generating system for airplanes.
2. Description of the Prior Art
Recently, with the progress in the practical use of a socalled VSCF (variable speed constant frequency) generating system which uses engine power as a driving source, the need for using a brushless generator as a brushless motor has been increased.
In a prior art VSCF generating system of the engine start type, it is usual to include separately a starter for starting the engine, and as shown in FIG. 1, such a system includes a starter 6 such as an air turbine or the like, for starting an engine 1, and an AC generator 2 is coupled to the engine 1. The AC output from the AC generator 2 is converted to desired AC power, for example, of three-phase, 115 V at 400 Hz through a power rectifier 3, a power inverter 4, and a filter 5.
Furthermore, a generating system of the DC excitation type is known, for example, from "Brushless Generator for Aircraft" by A. W. Ford, the Institute of Electrical Engineers Paper No. 3812 U, 1962, in which, as shown in FIG. 2A, this system includes a main generator 2 having a field winding 2a and an armature winding 2b, an AC exciter 9 having a field winding 9a and a rotor winding 9b, a DC power source 71, a DC controller 81, and a rotary rectifier 10. A rotor assembly K1 includes the rotor winding 9b, rotary rectifier 10, and field winding 2a.
In addition, a generating system of the AC excitation type is known, for example, from "Brushless Excitation with Rotating Transformer", SHINKO DENKI Technical Bulletin, Vol. 16, No. 2, 1971, in which as shown in FIG. 2B, and AC power source 72, an AC controller 82, and a rotary transformer 11 are provided to excite a field winding 2a of a main generator 2 by AC power through the rotary transformer 11 and a rotary rectifier 10. In this case, a rotor assembly K2 includes a secondary winding of the rotary transformer 11, the rotary rectifier 10, and the field winding 2b.
Specifically, the AC power supplied from the AC power source 72 is regulated by the AC controller 82 to an appropriate AC voltage according to a required torque at the time of starting, and the AC voltage is applied to the rotary transformer 11, the output thereof being rectified by the rotary rectifier 10 to excite the field winding 2a. In generation mode, AC power generated by a magnet generator (not shown) is regulated by the AC controller 82 so that an AC voltage which enables the main generator 2 to generate a constant voltage is applied to a primary winding of the rotary transformer 11.
However, the following problems are involved in the prior art systems.
In the system shown in FIG. 1 in which the starter 6 constituted by an air turbine or the like is separately provided, it is necessary to provide such an additional device (starter) as compared to the system used in airplanes wherein the DC power is primary electrical power and a generator for supplying power to various facilities in the airplane is used also as a DC motor serving as a starter for starting the engine. The necessity of such an additional device in particular poses a serious problem when the generating system is to be used on airplanes in which the reduction of weight is required.
In the DC excitation system shown in FIG. 2A, there has been the problem in that when the rotational speed of the rotor assembly K1 is zero, the electrical power is not generated in the exciter rotor winding 9b and therefore magnetic flux is not generated in the field winding 2a of the main generator 2.
On the other hand, in the AC excitation type system shown in FIG. 2B, since the rotary transformer 11 has no power amplifying capability, the stator and rotor require substantially equal capacity. As a result, although the field magnetic flux can be obtained at the time of starting, it is necessary to supply large electrical power to the rotary transformer 11 as compared to the exciter. Therefore the drawback is involved in that the size and weight is large as compared to the DC excitation type system.